1. Field of the Invention
The invention in general relates to the fabrication of metal oxides utilizing a precursor liquid applied to a substrate, and more particularly to a fabrication process in which the metal-oxygen-metal bonds of the final desired material are largely formed in the precursor liquid prior to the application of the liquid to the substrate.
2. Statement of the Problem
Metal oxides are well known to be useful as ferroelectrics, high dielectric constant materials, and to have many other applications. Recently, there has been much research directed toward using metal oxides in thin film applications, such as integrated circuits. However, their commercial use in such applications has been relatively limited up to the present time. To a significant extent, this is due to the difficulty of forming high quality thin films with precisely controlled composition.
Metal oxide films have perhaps most frequently been formed by sputtering. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IDEM(International Electron Devices Meeting) Technical Digest, Dec. 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. Other fabrication methods include pulsed laser deposition, and rapid quenching as listed in Joshi, P. C. et al., "Structural and Optical Properties of Ferroelectric Thin Films By Sol-gel Technique," Appl. Phys. Lett., Vol 59, No. 10, Nov. 91. All of the above methods are relatively violent processes and thus inherently result in relatively poor control of the composition of the final thin film as a whole and variable composition throughout the film. To better control the composition, methods in which a organic liquid precursor is applied to the film and then decomposed to form the metal oxide have been developed. One such method comprises the application of a sol-gel to a substrate followed by heating which decomposes the sol-gel and drives off the organics to form the metal oxide. See for example, U.S. Pat. No. 5,028,455 issued to William D. Miller et al., the Joshi article cited above, and B. M. Melnick, et al., "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990). In another method, what has been termed a "MOD" solution is applied to a substrate followed by heating which decomposes the MOD solution and drives off the organics to form the metal oxide. See "Synthesis of Metallo-organic Compounds for MOD Powers and Films", G. M. Vest and S. Singaram, Materials Research Society Symposium Proceedings, Vol. 60, 1986 pp. 35-42 and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", J. V. Mantese, A. L. Micheli, A. H. Hamdi, and R. W. Vest, in MRS Bulletin, Oct. 1989, pp. 48-53. In each of these prior art processes, the word "precursor" is used with two different meanings. Each process includes precursors for each individual metal, which precursors we shall call "initial precursors" herein. For example, in the first Vest paper referenced above, barium neodeconate is listed as the initial precursor of choice for the metal barium while bismuth 2-ethylhexanoate is listed as the initial precursor for the metal bismuth. The initial precursors are then dissolved in a common solvent to form a "final precursor" which contains all of the metals of the desired final thin film. Generally the sol-gel method utilizes metal alkoxides as the initial precursors, while the MOD technique utilizes metal carboxylates as the initial precursors. One sol-gel reference, the Miller patent referenced above, mentions one metal carboxylate, lead tetra-ethylhexanoate, as a possible precursor, however does not disclose how this may be used as a sol-gel, and furthermore rejects this precursor as less desirable since the large organic group was thought to result in more defects in the final film. The above liquid precursor methods produce a much better quality film than the previous more violent methods, since the metal and oxygen atoms are relatively uniformly distributed over the substrate. However, in most of the above processes, the metals in the precursor solution are linked by organic ligands, which ligands must be broken down and removed during the heating process. This creates relatively large distances across which the metal and oxygen atoms must link. This often results in cracking or other imperfections in the film, or requires careful control of other parameters, such as film thickness, drying and annealing temperatures, the substrate used etc. In other liquid processes, such as the sol-gel process described in Melnick, the metal-oxygen-metal bonds of the final metal oxide are present in some degree, however the precursor is highly unstable and therefore is difficult to use except immediately after preparation in the laboratory. Thus it would be highly desirable to have a fabrication process in which the constituents can be carefully controlled as in the sol-gel and MOD processes, and at the same time the metal and oxygen atoms are more closely associated prior to the formation of the final desired film and the precursors are sufficiently stable to be used in commercial manufacturing processes.
3. Solution to the problem
The present invention solves the above problem by utilizing a mixed alkoxide/carboxylate initial precursor. Unlike the prior art which utilizes either a metal alkoxide or a metal carboxylate as the individual precursor for a metal, the invention utilizes a alkoxycarboxylate as the individual precursor for at least one metal.
In an exemplary embodiment or the invention a first metal is reacted with an alcohol and a carboxylic acid to produce a metal alkoxycarboxylate initial precursor. Then the alkoxide of a second metal is added to the alkoxycarboxylate and reacted to form a final precursor containing both the first and second metals. In this final precursor the first and second metals are linked with a metal-oxygen-metal bond. A portion of the organics that remained in the prior art precursors until after application of the final precursor to the substrate are boiled out of the final precursor as ethers. Generally the organics which remain are alkoxide and carboxylate ligands which are linked to the metal oxide without significantly disturbing the metal-oxygen-metal bonds. The presence of the carboxylate ligands provide sufficient stability to the precursor against hydrolysis to permit it to be stored for months without significantly changing. During and/or after the application of the precursor to a substrate, these remaining organics are disassociated from the metal-oxygen-metal bonds, preferably by heating, thereby leaving the metal-oxygen-metal bonds in place.
In another embodiment a metal carboxylate is combined with a metal alkoxide and heated, preferable with the addition of carboxylic acid and/or alcohol. Many other variations of the process are possible.
Since a significant portion of the metal-oxygen-metal bonds of the final thin film are already formed in the final precursor, the resulting thin film is of higher quality and less susceptible to cracking and other defects than thin films formed with the prior art processes and precursors. In addition, since the metal and oxygen atoms are much more closely associated in the precursor, the quality of the film is less sensitive to the substrates and/or the process parameters used. Thus the precursors and process of the invention lend themselves more readily to large scale manufacturing. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.